With reference to FIG. 1, a ducted fan gas turbine engine generally indicated at 10 has a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high-pressure compressor 14, combustion equipment 15, a high-pressure turbine 16, and intermediate pressure turbine 17, a low-pressure turbine 18 and a core engine exhaust nozzle 19. A nacelle 21 generally surrounds the engine 10 and defines the intake 11, a bypass duct 22 and a bypass exhaust nozzle1 23.
The gas turbine engine 10 works in a conventional manner so that air entering the intake 11 is accelerated by the fan 12 to produce two air flows: a first air flow A into the intermediate pressure compressor 14 and a second air flow B which passes through the bypass duct 22 to provide propulsive thrust. The intermediate pressure compressor 13 compresses the air flow A directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high-pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines 16, 17, 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate pressure compressors 14, 13 and the fan 12 by suitable interconnecting shafts.
Leakage in gas turbines is a significant source of inefficiency and flap seals are regularly used to provide a fluid seal between adjacent components of the turbine wall. FIG. 2 shows the use of a single primary plate or flap seal 1 to provide a fluid seal between adjacent components 2, 4. However a single plate arrangement such as that shown in FIG. 2 results in gaps 3 needing to be left between the plates 1 to allow for movement and/or thermal expansion. These gaps can result in significant leakage.
A known way of overcoming this leakage is shown in FIG. 3, and uses overlap plates 5 (secondary plates) which cover the gaps between the primary plates 1. Often springs are used in conjunction with this type of seal in order to load the primary plate 1 against the adjacent components 2 and 4.
Generally, it is desirable for the seal contact faces of the primary plates 1 on the adjacent components 2 and 4 to be axially aligned so that the flap seal is vertical when in its working position. However, inherent build tolerances and differing thermal expansions mean that this objective is rarely achieved in all design conditions and a crescent shaped leakage gap arises as shown in FIG. 4 which shows a cross-sectional and isometric view of a seal created by a primary plate 1 between adjacent components 2, 4 between which an axial misalignment 52 has resulted. As shown in FIG. 4, crescent shaped gaps 51 arise at the centre of the primary plate 1 due to this misalignment. In many situations this gap is forced shut by the momentum and pressure effects of the gas in the turbine. In other arrangements closure of this gap may be assisted by a mechanical spring.
For the primary plate 1, the force required to load the plate into a sealed position (and thereby close the crescent shaped gap) can be sufficient even when there is a low pressure differential across the seal due to the relatively large surface area of the primary plate. Similarly the relative length of the primary plate means that the force required to close any crescent shaped gap in the vicinity of the primary plate using a mechanical spring is also relatively low. However, by design, the overlap plate 5 typically spans a much shorter length and seals over a gap of much smaller area. This potentially results in a situation in which neither the overpressure of the gas in the turbine or the force of a mechanical spring can generate sufficient force to close any crescent shaped gap that arises in the vicinity of the overlap plate 5.
Therefore it is desirable to achieve the best possible seal when overlap plates are used as flap seals and in similar arrangements.